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 The Ultimate in FPC Surface Finish Solutions of 5 Types

The Ultimate in FPC Surface Finish Solutions of 5 Types


At Gesp, we are constantly at the forefront of innovation in flex PCB manufacturing. We offer our expertise in the manufacturing of FPCs and rigid-flexible PCBs. As we know, FPC Surface finish plays a pivotal role in the functionality of Flexible PCBs. It’s not just about aesthetics; it’s about enhancing reliability and protecting the circuit. and ensuring consistent performance. In this article, we will introduce in detail the importance. and methods of surface treatment in flexible circuit boards.

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What is The FPC Surface Finish ?

The term “FPC surface finish” refers to a critical aspect of the fabrication process. that significantly influences the performance, durability, and reliability of flexible printed circuits (FPCs). Surface finish is the final coating applied to the exposed copper traces on an FPC. serving various essential purposes to ensure the circuit’s functionality and longevity. The Role of FPC Surface Finish:

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※ Protection:

The primary function of FPC surface finish is to protect the underlying copper traces. such as moisture, dust, and chemical contaminants. Without this protective layer, the copper traces would be susceptible. to corrosion and oxidation, leading to a degradation of electrical performance.

※ Solderability:

Surface finish is crucial for facilitating the soldering of electronic components to the FPC. It provides a solder-friendly surface. ensuring a secure and reliable connection between the components and the circuit.

※ Electrical Performance:

Different surface finishes can impact the electrical characteristics of the FPC. including impedance, signal integrity, and conductivity. Manufacturers choose specific surface finishes based on the required electrical properties.

※ Durability and Reliability:

The surface finish enhances the overall durability and reliability of the FPC. It protects the circuit against mechanical stress and repeated flexing. and the wear and tear it may encounter during its operational life.

※ Environmental Resistance:

Certain surface finishes offer resistance to harsh environmental conditions. making FPCs suitable for applications in industries. such as aerospace, automotive, and medical devices.

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5 Types of FPC Surface Finish

1. Immersion Gold (ENIG)

Immersion Gold (ENIG) is a surface finish that involves a two-step process to protect. and enhance the copper traces on a flexible printed circuit. The first step includes depositing a thin layer of nickel onto the copper traces. followed by the deposition of a layer of gold. This creates a flat, planar surface, offering numerous advantages for FPCs.

Key Benefits of ENIG Surface Finish:

  • Advantage: ENIG Surface Finish is not easy to oxidize, and stored for a long time, flat surface. Suitable for soldering precision pins and components with small solder joints. suitable for reflowing repeatedly without reducing solderability.
  • Outstanding Solderability: ENIG owned excellent solderability, making it a top choice for FPCs. It ensures a reliable and strong bond between electronic components and the circuit.
  • Corrosion Resistance: The gold layer acts as a protective barrier. preventing copper traces from corroding or oxidizing, which is crucial for long-term reliability.
  • Flat Surface: ENIG provides a smooth, flat surface. which is essential for attaching fine-pitch components, where precise soldering is paramount.
  • High Signal Integrity: This surface finish maintains excellent electrical characteristics. including impedance control and signal integrity, making it suitable for high-frequency applications.
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High cost, poor welding strength, and easy to appear pad black problem. due to the chemical nickel plating process. Over time, the nickel layer oxidizes, making long-term reliability a problem.

2. Immersion Silver

Immersion Silver is a surface finish used in the fabrication of FPCs. It involves depositing a thin layer of silver onto the exposed copper traces of the circuit. to protect and enhance the performance of the FPC. Here are some of the benefits of using Immersion Silver in FPC fabrication:

Key Benefits of Immersion Silver Surface Finish:

  • Advantages:Simple process, suitable for lead-free soldering and SMT. very flat surface, low cost, suitable for very critical circuits.
  • Good Solderability: Immersion Silver provides excellent solderability. This means that it ensures strong and reliable solder joints. between electronic components and the FPC. When components soldered onto the FPC during the assembly process. Immersion Silver promotes a solid and secure bond.
  • Cost-Effective: Immersion Silver is a cost-effective surface finish option . like gold or ENIG (Electroless Nickel Immersion Gold). This makes it a preferred choice for projects with budget constraints. or when cost-efficiency is a key consideration.
  • Flat Surface: Immersion Silver creates a relatively flat and even surface on the FPC. This flatness is essential for the precise soldering of fine-pitch components. and the mounting of components with high density, where soldering precision is crucial.
  • Reduced Surface Oxidation: The thin silver layer provides protection against surface oxidation. which is vital for maintaining the integrity and reliability of the FPC over time.


High storage requirements, susceptible to contamination. Solder strength prone to microcavity problems. Prone to electromigration and galvanic corrosion on solder masks. also not favorable for electrical measurements.

3. OSP (Organic Solderability Preservatives)

OSP stands for Organic Solderability Preservatives. That is a surface finish technique designed to protect the exposed copper traces on an FPC. Unlike some other finishes, OSP is organic and water-based. making it an environmentally friendly choice. It provides a protective layer that ensures solderability and corrosion resistance.

Key Benefits of OSP Surface Finish:

  • Advantages: simple process, very flat surface, suitable for lead-free soldering and SMT. easy rework and production, suitable for horizontal operation. Low cost and environmentally friendly.
  • Solderability: OSP surface finish offers excellent solderability. making it easier to attach electronic components to FPCs during the assembly process. It forms a uniform and reliable soldering surface.
  • Environmentally Friendly: OSP is an eco-friendly option due to its water-based organic nature. making it a responsible choice for environmentally conscious projects.
  • Cost-Effective: OSP is often more cost-effective compared to finishes like gold or silver. making it an excellent choice for projects with budget considerations.
  • Flat and Planar Surface: OSP provides a flat and planar surface. ensuring that fine-pitch components can be precisely soldered. which is crucial for modern electronics.


Reflow soldering limited, and not suitable for crimping technology, or lead bonding. Visual inspection and electrical testing are not convenient. SMT requires nitrogen protection, not suitable for SMT rework. High storage condition requirement.

4. HASL (Hot Air Solder Leveling):

HASL, or Hot Air Solder Leveling, is a surface finish method that involves dipping an FPC in molten solder and then using hot air to remove excess solder, leaving a thin, even layer on the exposed copper traces. This results in a solderable surface that ensures component attachment.

Key Benefits of HASL Surface Finish:

  • Advantages: shelf life > 12 months. Suitable for contact switch design and gold wire bonding as well as electrical testing.
  • Robust Solderability: HASL offers robust solderability, ensuring secure and reliable solder joints between electronic components and the FPC, making it suitable for a wide range of applications.
  • Cost-Effective: This surface finish is often cost-effective, making it a competitive choice for projects with budget constraints.
  • Flat Surface: HASL provides a relatively flat and even surface, allowing for accurate and consistent soldering, which is important for high-density and fine-pitch component attachment.
  • Versatility: HASL can be applied to various FPC applications, especially those that do not require the ultra-thin profiles and extreme flexibility of some other surface finishes.


Higher cost and thicker gold. Additional circuit design required to conduct electricity when plating gold fingers. Due to inconsistent gold thickness. soldering may result in brittle solder joints due to too thick gold. Plating surface uniformity issues. Nickel-plated gold will not wrap around the edges of the wire.

5 Immersion Tin :

Immersion Tin is a surface finish technique applied to FPCs. It involves depositing a thin layer of tin onto the exposed copper traces of the circuit. creating a protective, solderable, and flat surface that offers numerous advantages.

Key Benefits of Immersion Tin Surface Finish:

  • Advantages: Suitable for horizontal production. Suitable for precision alignment processing. suitable for lead-free soldering, especially suitable for crimping technology. Very good leveling, suitable for SMT.
  • Solderability: Immersion Tin well known for its excellent solderability. It ensures strong solder joints, making it ideal for attaching electronic components.
  • Cost-Effective: It is often a cost-effective choice compared to finishes like gold or silver. This makes it a preferred option for projects with budget constraints.
  • Flat and Planar Surface: The finish provides a smooth and planar surface. essential for precise soldering, especially for fine-pitch and densely populated component designs.
  • Environmental Resistance: While not as corrosion-resistant as gold. Immersion Tin offers protection against environmental factors. and ensures the long-term reliability of FPCs.


Requires good storage conditions, no more than 6 months, to control tin whisker growth. Not suitable for contact-switching designs. The Soldermask process requires high workmanship, otherwise, the soldermask will come off. It is best to protect with nitrogen when soldering multiple times. Electrical measurement is also a problem.

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Choosing the right surface finish for a Flexible PCB (FPC) is a crucial decision. that can significantly impact the performance, cost, and reliability of your FPC. To make the right choice, follow these steps:

  • Understand Your Application Requirements:
  • Evaluate Surface Finish Options
  • Consider Environmental Factors:
  • Think About Component Soldering:
  • Review Cost Implications:

By carefully considering these factors, you can make an informed decision. when selecting the right surface finish for your FPC. ensuring that it meets the specific needs and performance requirements of your project.

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